Galvanic cell having rechargeable zinc electrode

ABSTRACT

A galvanic cell comprising at least one rechargeable zinc electrode as anode, at least one other electrode as cathode, an electrolyte, and at least one foraminous electrically conductive auxiliary structure interposed between every anode and cathode. The auxiliary structure is galvanically separated from the electrodes of the cell and macroscopically visible electrolyte zinc deposition from the cell electrolyte thereon is only observed at potentials which are more negative than on a smooth nickel structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 576,023 filedMay 9, 1975 now abandoned, which application is a continuation ofapplication Ser. No. 375,818 filed July 2, 1973, and now abandoned.

This invention relates to a galvanic cell having a rechargeable zincelectrode.

In conventional galvanic cells having rechargeable zinc electrodes,which can include, for example, zinc/air, zinc/silver oxide, orzinc/nickel oxide electrodes, the lifetime of the cell is undesirablylimited by internal short circuits. These short circuits are producedduring the charging of the cell due to the fact that the zinc from theelectrolyte is not uniformly deposited on the anode. The deposit of zincincreases at preferred locations at a relatively high speed in thedirection toward the cathode. The cause of this is to be seen in thedepletion of zinc ion from the electrolyte with progressive charging.The depletion of the electrolyte in zinc ions leads to the formation ofparticularly rapidly growing zinc dendrites and, in certain cases, dueto differences in concentration, to convex bulges of the electrodewhich, just as the dendrites, result in an internal short-circuiting ofthe cell after a few cycles.

Attempts have been made to avoid this short-circuiting by the use ofseparators. The conventional separators can be grouped, according totheir mode of operation, into those which are to prevent mechanically apenetration of the zinc deposits to the cathode and furthermore intothose which effect a precipitation of the zinc ions in the form of zinccompounds insoluble in the electrolyte. However, mechanically effectivedendrite barriers do not prolong the lifetime of the cell to asubstantial degree, inasmuch as the dendrites can grow through theseparator. Therefore, efforts have been expended, in turn, to constructthis type of separator in such a way that the diffusion of the zinc ionsthrough the separator is made difficult. This, however, requires a highinternal resistance of the separators due to the necessary smallness ofthe pores formed therein.

The chemically effective barriers are based on the precipitation of thezinc ions by alkaline earth hydroxides in the form of alkaline earthzincates. However, this type of separator, due to the solubility of thesolid phases, demands the use of relatively dilute electrolyte liquor,connected, in turn, with an undesirably high internal resistance of thecell. Also, an excess of alkaline earth hydroxide is required for thequantitative precipitation of the zincate in a limited reaction zone,having a disadvantageous effect on weight and volume of the cell.

Another type of separator provides that the zinc dendrites are removedby oxidation with oxygen dissolved in the electrolyte. In order toaccomplish this, the separator must have electro-catalytic propertiesfor the oxygen reduction. Therefore, carbon felts or thinlysilver-plated carbon felts are suggested preferably as catalyticmaterials for such separators. However, due to the low solubility ofoxygen in alkaline solutions, only very low charging current densitiesare possible, at which densities the dendrite growth is suppressed;anyway.

A further type of separator provides a combination ofdiffusion-inhibiting diaphragms with electrolyte-absorbing fibrous matsand a layer of sintered nickel which is microporous and -- which isessential -- partially rendered hydrophobic. This arrangementrepresents, on the one hand, a mechanical dendrite barrier and, on theother hand, serves the purpose of exploiting the ignoble character ofthe zinc which, as is known, is dissolved upon contact with metals ofsmall hydrogen overvoltage in strong electrolytes in accordance with:

    Zn + 2H.sub.2 O + 2 OH.sup.- → Zn(OH).sub.4.sup.2- + H.sub.2

with hydrogen evolution. The feature of rendering the sintered nickellayer hydrophobic serves the purpose of making it difficult for the zincions to be adsorbed, which adsorption precedes the disposition of themetallic zinc.

The disadvantages of this separator system reside, in particular, in itscomplicated multiple-layer construction of microporous materials, thuscausing an undesirably high internal resistance of the cell. Ifdendrites come into contact with the sintered nickel layer in spite ofthe diffusion-inhibiting diaphragms, a dissolution of the dendrites canbe accomplished only under hydrogen evolution. In accordance with theabove reaction scheme, two hydrogen molecules are produced in thisprocess for each dissolved zinc atom. The thus-produced gaseoushydrogen, of necessity, increases the internal resistance furthermorevigorously in the pores of the sintered nickel layer and can only beremoved gradually, due to the structure of the separator system, which,in turn, means a limitation to low charging current densities. Moreover,a disadvantageous consequence of rendering the sintered nickelhydrophobic is that the reaction according to the above equation andthus the evolution of hydrogen take place only to a subordinate extent.

It is an object of this invention to provide a galvanic cell of a simpleconstruction with a rechargeable zinc electrode, which, by exhibiting alow internal resistance, permits comparatively high charging anddischarging current densities, and has a capacity and degree of chargingefficiency that remain substantially constant over a large number ofcycles. This object is attained, according to this invention, byproviding that the zinc electrode is surrounded, in an at least partialmanner, by at least one foraminous, electrically conductive auxiliarystructure or means, galvanically separated from the electrodes of thecell. On this structure, a macroscopically visible electrolytic zincdeposition takes place from the cell electrolyte only at potentialswhich are more negative than the potential at which deposition occurs onsmooth nickel.

The auxiliary structure is galvanically separated from the anode andcathode of the cell during discharging and at the beginning of thecharging process, wherein the separation can be effected by mountingspacer pieces in the cell housing, or also by means of coarse-meshstructures of a nonconducting material. A galvanic contact with one ofthe electrodes can only take place from the side of the zinc electrodeby growth of the zinc. However, since the deposition of zinc isimpossible even at a high current density, any dendrites which areformed are dissolved and, in case of a massive contacting of zinc, thereoccurs a large-area oxidation of the zinc under hydrogen liberation. Thethus-produced hydrogen escapes rapidly, due to the coarse-poredcharacter of the structure.

By means of an arrangement according to this invention, the charging ofa zinc-nickel oxide cell is made possible, since for nickel oxidecathodes, part of the charging current is always converted for oxygenevolution and, accordingly, for cycle operation, it is necessary toadapt the degree of charging efficiency of the zinc electrode to theefficiency of the nickel oxide cathode. This adaptation is obtained, inaccordance with the invention, by zinc oxidation on the auxiliarystructure. For, once the zinc deposition produced during charging hasreached a certain thickness on the anode, these deposits will contactthe auxiliary structure -- even if no intermediate or temporary dendriteformation has occurred. Any current used for charging purposes afterthis point in time is utilized for hydrogen evolution, so that, inaccordance with a further aspect of this invention, the degree ofcharging efficiency of the zinc electrode can be regulated by thestrength and duration of the charging current and the capacity of thezinc electrode can be regulated by the distance between the anodestructure and the auxiliary structure. In this connection, it is anadvantageous and essential feature that the active material of thestructures be well wettable.

Preferably, the surface of the auxiliary structures contains substancesor mixtures of substances usable as electro-catalysts for the hydrogenoxidation in fuel cells with an aqueous electrolyte, such as, forexample, Raney nickel, finely divided platinum, tungsten carbide, ortitanium-nickel alloys (e.g. 35 weight % to 85 weight % Ni). Contactingzinc is immediately oxidized by the electrolyte due to the extremely lowhydrogen overvoltage of these substances. Simple nickel plates orelements of sintered nickel are not suitable for this purpose.

Preferably, there will be no macroscopically visible deposition of zincon the auxiliary structure in case of potentials being more positivethan -1400 millivolts with respect to a mercury oxide referenceelectrode.

In an advantageous embodiment of the invention, the cell is providedwith a metallic net as the auxiliary structure, on which the activelayer has been applied by electroless chemical or galvanic deposition.Electrophoretic or mechanical methods can also be advantageouslyemployed for this application, wherein the thus-applied powder adheresto the structure by means of suitable binders or by a heat treatment.Furthermore, the active layer can be produced by deposition from thegaseous phase (i.e. by vapor deposition), by treatment with melts, or byetching. Suitable structures, in addition to coarse-meshed nets,perforated metal sheets, or expanded metal, include other coarse-meshedsystems, such as, for example, fibrous skeletons or several layers ofmesh, or open-pore foam elements.

The galvanic cell of this invention is illustrated in the accompanyingdrawings wherein:

FIG. 1 shows one embodiment of a cell having one anode and two cathodesand;

FIG. 2 shows another embodiment of the cell having two anodes and onecathode.

Normally, only one porous auxiliary structure, e.g. in the form of a netwith low hydrogen overvoltage is interposed between an anode and acathode. If an anode is surrounded on both sides by cathodes, one net orporous auxiliary structure is provided for each side. An electricalconnection between the nets can e.g., be made by using a single piece ofnetting and giving it a U-shaped configuration as illustrated in FIG. 1.Usually not more than two nets are provided on each side of the anode,but the range of the number of conductive auxiliary structuressurrounding every zinc electrode is from 1 to 4.

The thickness of the electrocatalytic coating provided on the nets orlike structures is from about 0.0005 to about 0.05 mm. and the thicknessof the entire auxiliary structure may vary from about 0.1 to about 1 mm.

It will be appreciated that the auxiliary structures of this inventionare intended for conventional alkaline storage batteries.

The invention will be further understood from the following examples:

EXAMPLE 1

A single cell, containing as the electrolyte a saturated zincatesolution in 6 M KOH, and having a zinc anode that was opposed on bothsides by nickel oxide cathodes was employed. This cell contained,between the anode and the cathodes two nickel nets having a mesh widthof 1 mm., on which a nickel-aluminum alloy, according to Raney, (i.e. analloy of 30 wt. % Ni and 70 wt. % A1) had been applied by a plasmaspraying unit. The nickel nets had been activated by subsequentlytreating the aluminum with lye (i.e. 6 M potassium hydroxide).

The cell was charged and discharged, respectively, in 45 minutes with 50milliamperes/cm² ; the capacity and the degree of charging efficiencyexhibited no insubstantial fluctuations over 100 cycles.

EXAMPLE 2

A zinc-nickel oxide cell of the type described in Example 1 contained,between the anode and the cathodes, two nickel nets having a mesh widthof 1 mm., on which a nickel-boron alloy (i.e. an alloy of 93 wt. % Niand 7 wt. % B) has been deposited by electroless reduction of a (1.7%)nickel chloride solution with sodium borohydride. This alloy is an x-rayamorphous, nickel boride. The cell was charged and discharged,respectively, in 45 minutes with 40 milliamperes/cm² ; the capacity andthe degree of charging efficiency again remained essentially constantover 100 cycles.

EXAMPLE 3

A single cell as illustrated in FIG. 1 was constructed, comprising ahousing 1, a flat planar zinc electrode 2 containing an admixture of 40weight % calcium hydroxide and 1 weight % mercuric oxide, two flatplanar nickel oxide electrodes 3,3' opposing both sides of the zincelectrode, two separators 4,4' consisting of coarse-mesh plasticnetting, a nickel net 5 surrounding the zinc electrode on both sides,having a mesh width of 0.5 mm. and coated with tungsten carbide byplasma spraying. Interposed between net 5 and electrode 2 was a standardmicroporous separator 6 and a coating 8 on the zinc electrode,consisting of calcium hydroxide supported by a polyamide felt. The wholeassembly was packed tightly together in the housing. The electrolyte waspotassium hydroxide solution with 22 weight % KOH.

The cell was charged with 600 milliamperes for 5 hours and dischargedwith 1500 milliamperes; the capacity and the charging efficiency againremained highly constant over 150 cycles.

EXAMPLE 4

A single cell as illustrated in FIG. 2 was set up, comprising a housing1; two flat planar zinc electrodes 2,2' held in grooves in the housing;two separators 4,4' of coarse nylon mesh; two catalytic nickel nets 5,5'of mesh width 1.5 mm. coated with a titanium-nickel alloy containing 50weight % alloyed nickel surrounding the nickel oxide electrode on bothsides and extending beneath the zinc electrodes; two standardmicroporous separators 6,6' interposed between the catalytic nets andthe zinc electrodes, spaced apart from the zinc electrodes and heldtightly against cell components 5,5'; 4,4' and 3 by winding severalturns of polyamide sewing thread around the vertical portions. The wholepacket containing components 3 to 6 was held in a groove in the housing.The electrolyte was 10 M potassium hydroxide saturated with zincateions.

The cell was charged with 1 ampere for 5 hours and discharged with 4amperes; again capacity and charging efficiency remained nearly constantfor 100 cycles.

While the novel embodiments of the invention have been described, itwill be understood that various omissions, modifications and changes inthese embodiments may be made by one skilled in the art withoutdeparting from the spirit and scope of the invention.

What I claim is:
 1. A galvanic cell comprising at least one rechargeablezinc anode, at least one cathode, alkaline electrolyte and at least oneforaminous, wettable, electrically-conductive, coarse-pored auxiliarystructure interposed between and galvanically separated from respectiveanodes and cathodes, said at least one auxiliary structure containing onits surface at least one electrocatalyst selected from the groupconsisting of Raney nickel, finely divided platinum, tungsten carbide,amorphous nickel-boron alloys and titanium-nickel alloys.
 2. Thegalvanic cell of claim 1, in which the at least one auxiliary structureconsists of a plurality of coated metallic nets.
 3. The galvanic cell ofclaim 2, in which the metallic nets have a mesh width of about 1 mm. 4.The galvanic cell of claim 1, in which the at least one auxiliarystructure comprises a plurality of auxiliary structures consisting ofcoated fibrous mats, non-wovens, or open-pored foam elements. 5.Galvanic cell according to claim 1, in which the zinc electrode isdisposed in a pocket closed on at least one side and fashioned as anauxiliary structure.
 6. The galvanic cell of claim 1, in which the atleast one auxiliary structure comprises auxiliary structures of one cellthat are connected conductively with one another.
 7. The galvanic cellof claim 1, in which zinc compounds insoluble in the electrolyte areprovided as bottom elements in the electrolyte chamber or in amechanical connection with the zinc anode.
 8. The galvanic cell of claim1, in which the cathode is an air electrode.
 9. The galvanic cell ofclaim 1, in which the cathode is a nickel oxide electrode.
 10. Thegalvanic cell of claim 1, in which the at least one auxiliary structureis fixedly joined to the zinc electrode.
 11. The galvanic cell of claim1 in which the at least one auxiliary structure is fixedly joined to thecathode.
 12. The galvanic cell of claim 1, in which the capacity of thezinc electrode, with a predetermined zinc concentration in theelectrolyte, can be adjusted by the distance of the at least oneauxiliary structure from the current discharge structure of the zincelectrode.
 13. The galvanic cell of claim 1, characterized in that thedegree of charging efficiency of the zinc electrode in the electrolytewhich contains zinc ions is adjustable by charging duration and chargingcurrent.
 14. The galvanic cell according to claim 1, wherein theopenings of the at least one coarse-pored auxiliary structure are ofsuch size that hydrogen formed at the structure is permitted to rapidlyescape therethrough.
 15. The galvanic cell according to claim 14,wherein a microporous separator is provided between said at least oneauxiliary structure and said at least one zinc anode.
 16. The galvaniccell according to claim 15, further comprising a coating of calciumhydroxide supported by a polyamide felt on said zinc anode.
 17. Thegalvanic cell of claim 16, further comprising an additional separatorprovided between said at least one auxiliary structure and said at leastone cathode, said additional separator comprising a coarse-mesh plasticnetting.
 18. The galvanic cell of claim 15, further comprising anadditional separator provided between said at least one auxiliarystructure and said at least one cathode, said additional separatorcomprising a coarse-mesh plastic netting.
 19. The galvanic cell of claim18, wherein a zinc anode is provided between a pair of nickel oxidecathodes.
 20. The galvanic cell of claim 1, wherein said at least oneauxiliary structure is interposed between each pair of anodes andcathodes.
 21. The galvanic cell of claim 1, wherein said at least oneauxiliary structure comprises 1 to 4 auxiliary structures that surroundsaid at least one zinc anode.
 22. The galvanic cell of claim 1, whereinsaid at least one auxiliary structure is formed from a conductive nethaving an electrocatalyst coating thereon, the thickness of theelectrocatalyst coating being about 0.005 to about 0.05 mm.
 23. Thegalvanic cell of claim 22, wherein the thickness of the auxiliarystructure is about 0.1 to 1 mm.
 24. The galvanic cell of claim 1,wherein the thickness of the auxiliary structure is about 0.1 to 1 mm.25. A galvanic cell comprising at least one rechargeable zinc anode, atleast one cathode, alkaline aqueous electrolyte, and at least oneforaminous, wettable, electrically conductive, coarse-pored auxiliarystructure interposed between and galvanically separated from respectiveanodes and cathodes, said at least one auxiliary structure containing onits surface at least one to metal-containing electrocatalyst that hassuch low hydrogen overvoltage that there is no visible zinc depositionon said auxiliary structure during charging and discharging at a currentdensity of at least 40 milliamperes/cm² whereby contacting zinc isoxidized and dissolved in the electrolyte and whereby hydrogen isgenerated during the oxidation, said hydrogen escaping from the at leastone coarse-pored auxiliary structure.
 26. The galvanic cell according toclaim 25, wherein the openings of the coarse-pored at least oneauxiliary structure are of such size that hydrogen formed at thestructure is permitted to rapidly escape therefrom.
 27. The galvaniccell according to claim 26, wherein a microporous separator is providedbetween said at least one auxiliary structure and said at least one zincanode.
 28. The galvanic cell according to claim 27, wherein saidmicroporous separator is a polyamide felt.
 29. The galvanic cell ofclaim 27, further comprising an additional separator provided betweensaid at least one auxiliary structure and said at least one cathode,said additional separator comprising a coarse-mesh plastic netting. 30.The galvanic cell of claim 29, wherein a zinc anode is provided betweena pair of nickel oxide cathodes.
 31. The galvanic cell of claim 25,wherein said at least one auxiliary structure is interposed between eachpair of anodes and cathodes.
 32. The galvanic cell of claim 25, whereinsaid at least one auxiliary structure surrounds said at least one zincanode.
 33. The galvanic cell of claim 25, wherein said at least oneauxiliary structure is formed from a conductive net having saidelectrocatalyst coating thereon, the thickness of the electrocatalystcoating being about 0.005 to about 0.05 mm.
 34. The galvanic cell ofclaim 33, wherein the thickness of the auxiliary structure is about 0.1to 1 mm.